In recent years, demands for reduction in electric power consumption of display devices, reduction in thickness and increase in flexibility grow larger as information devices advance, and research and development of display devices which meet the above demands are actively carried out.
An electrophoretic display device invented by Harold D. Lees et al. is known as one of the above display devices (refer to, for example, patent document 1). The electrophoretic display device has a structure in which two electrode substrates, out of which at least one is transparent, are oppositely disposed via suitable spacers and in which a display liquid prepared by dispersing fine particles (pigment particles) in a dispersion medium colored differently from the color of the fine particles is filled into a space between the above electrode substrates to prepare a display panel, and display is shown on the transparent electrode face by applying an electric field to the display panel.
A liquid for electrophoretic display filled between the electrode substrates is constituted from fine particles of titanium oxide and the like (pigment particles), a dispersion medium having a low dielectric constant such as xylene, tetrachloroethylene, paraffin, silicone oil in which a dye for providing contrast of a color against the fine particles is dissolved, a dispersant such as a surfactant and additives such as an electric charge-providing agent. The fine particles in the display liquid transfer to a transparent electrode side by applying an electric field to the liquid for electrophoretic display to allow the color of the fine particles to appear on the display face. Further, the fine particles transfer to a reverse side by applying an electric field in a direction which is reverse to the above to allow the color of the dispersion medium colored by the dye to appear on the display face.
The above electrophoretic display device is a display device in which desired display can be obtained by controlling a direction of an electric field, and it has advantages that it provides a low cost and has a viewing angle which is as broad as those of printed matters and that it is small in electric power consumption and has a memory property of display, so that it is paid attention as an inexpensive display device.
Since the display liquids for the above electrophoretic display devices is prepared usually by dispersing an inorganic pigment having a high refractive index such as titanium dioxide in a hydrophobic dispersion medium having a low dielectric constant which is colored by dissolving therein a dye, it is difficult to control dispersion stability thereof, and the liquid has defects that coagulation of the pigment brings about reduction in the contrast and the repetitive display characteristics and reduction in the display quality.
Further, since the dye solution is used, it has the defect that the contrast is reduced by adsorption of the dye onto the pigment surface. Accordingly, a lot of proposals for stabilizing the dispersion systems and improving the display characteristics have so far been made in order to solve the above problems.
For example, a technique in which sorbitan fatty acid ester surfactants are added to a system using electrophoretic particles subjected to surface treatment with a titanate coupling agent in a dispersion medium colored by a dye is proposed as a means for raising the dispersion stability (refer to, for example, patent documents 2 and 3).
However, an effect of the stabilization of dispersion has not yet reached a satisfactory level, and room for study is present still in the contrast of the display.
On the other hand, it is proposed as a means for improving the contrast that a dye which is non-adsorptive to a pigment surface is used as a dye used for coloring a dispersion medium and that a dye concentration in a dispersion medium is reduced.
However, the methods do not come to completely solve the defects brought about by the dye solutions described above and still have problems in terms of practical use.
Further, a method in which a microcapsule is filled with a display liquid for electrophoretic display and used as a display particle is proposed as a method for preventing unequal display caused by uneven distribution of electrophoretic fine particles on a display face (refer to, for example, patent document 4).
However, also in the method, a colored dye solution and a dispersion liquid of pigment particles are used in a display liquid filled, and therefore a problem resides in that it is not satisfactory in terms of contrast as is the case with the phenomenon described above.
Then, an electrophoretic display device in which a dye solution is not used is proposed as a means for solving the defects involved in the systems in which a dispersion medium colored by a dye is used. Proposed is, for example, an electrophoretic display device in which a liquid prepared by dispersing at least two kinds of electrophoretic fine particles different in a color tone and an electrophoretic property from each other in a colorless dispersion medium is filled in a cell formed via spacers between two oppositely disposed electrodes out of which at least is transparent (refer to, for example, patent document 5).
However, electrified charge of the electrophoretic fine particles having different color tones is combination of a positive charge and a negative charge, and therefore involved therein is the problem that an electric attracting force is produced between the fine particles to cause coagulation of the fine particles, whereby the dispersion stability is reduced to bring about reduction in the contrast by mixed color.
Also, use of a steric repulsion effect brought about by addition of a charge-controlling agent and surface treatment of particles is known as a means for preventing coagulation between particles which is a problem in a system using, as a display liquid for electrophoretic display, the above liquid prepared by dispersing two kinds of the electrophoretic fine particles which are different from each other in a color tone and an electrophoretic property (electrified charge) (refer to, for example, patent document 6). In this case, however, involved therein is the problem that it is difficult as well to completely prevent coagulation of two kinds of the electrophoretic fine particles and that good contrast can not be actualized.
Further, proposed is an electrophoretic display device prepared by using a liquid dispersed at least two kinds of electrophoretic fine particles which have the same electrophoretic property and which are different in a color tone and an electrophoretic speed from each other in a colorless dispersion medium (refer to, for example, patent document 7). In the electrophoretic display device, however, even the particles moving at a slow speed move only a shorter distance when present close to the electrode on the display face and therefore appear on the display face in a certain case. Also, an electrified charge is usually different even between particles having the same color tone, and the moving speed thereof has distribution. Accordingly, involved therein is the problem that it is difficult to obtain the satisfactory contrast without controlling so that they are not superposed among the particles having different color tones.
Further, proposed is a display liquid for electrophoretic display containing in a dispersion medium particles which are subjected to surface treatment and have voids in the inside and pigment particles which are different in a color tone from the above particles (refer to, for example, patent document 8).
However, in the above display liquid for electrophoretic display, the dispersion medium enters into the voids of the particles, and therefore particularly the white particles are reduced in a refractive index to make it difficult to obtain the satisfactory contrast.
In addition, proposed as well is a case in which a liquid prepared by dispersing at least two kinds of electrophoretic fine particles which are different in a color tone and an electrophoretic property from each other in a colorless dispersion medium is filled in microcapsules (refer to, for example, patent document 9). However, involved therein are the problems that the contrast is weakened by a resin layer of the microcapsule and that unevenness is formed in contrast on the display face by having the curved face part.
On the other hand, known is a suspension for electrophoretic display containing colored, charged electrophoretic particles absorbing light, transparent particles transmitting light and a dispersion medium for dispersing the colored, charged electrophoretic particles and the transparent particles for the purpose of providing an electrophoretic display device having an electrooptical characteristic of high contrast and a displayed image memory property after applying an electric field, wherein a refractive index n1 of the transparent particles is 1.4n2≧n1≧0.86n2 based on a refractive index n2 of the dispersion medium (refer to, for example, patent document 10).
However, the suspension for electrophoretic display disclosed in the patent document 10 reduces the effective aperture rate in light display and is completely different in subjects and technical concepts from the present invention (constitution, effects thereof and the like).